The selection of an appropriate filter type will be based on considerations of minimizing the pressure drop for a desired air flow rate while removing very small particles. It may also be influenced by the types of filters that local suppliers actually have available.
At laboratory scale, contamination is typically somewhat easier to control while at large scale sterilization and prevention of contamination is more complicated. In fact, contamination may be one of the most frequent operational problems for those processes that require aseptic operation of the bioreactor. During the production of gibberellic acid by Gibberella fujikuroi in the 200-kg capacity pilot scale bioreactor of PUC, Chile (see Sect. 10.3.1.3 and Fig. 10.4) there were frequent contamination problems when the air system contained only a pre-filter and an absolute filter with a cut-off of 0.3 |im. These contamination problems were reduced significantly by (1) the installation of UV lamps in the air duct between the filter system and the bioreactor and (2) chemical sterilization of the air duct system prior to each fermentation run.
In designing an appropriate system, several factors will need to be considered, such as capital and operating costs, the effectiveness of removal of microorganisms, the potential for failures in the system (such as the rupturing of filters), and the pressure drop contributed by the air sterilization system. It is also important to consider at what stage of the air preparation system the sterilization should occur. If the air is dry at the time of sterilization, then there should be no problem of wetting of filters, however, any subsequent humidification steps will need to be done aseptically if aseptic bioreactor operation is required for the particular process in question.
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